Gas hydrates (or clathrate hydrates) are crystalline water-based solids which physically resemble ice and in which small non-polar molecules, partially polar molecules or polar molecules with large hydrophobic moieties, such as methane and carbon dioxide, are trapped inside cage-like structures of hydrogen bonded water molecules. The molecules trapped in the cage-like structures lend support to the lattice structure of the gas hydrate through van der Waals interactions; without such support the lattice structure is liable to collapse into a conventional ice crystal structure or liquid water. Gas hydrates typically form under elevated pressure and low temperature conditions. Such gas hydrate formation favouring conditions often arise in oil/gas pipelines and may result in agglomerations of clathrate crystals which are liable to obstruct the flow line, limit or stop production and/or damage equipment, such as pipelines, valves and instrumentation, and thereby pose significant economic and safety concerns. The formation of gas hydrates in oil and gas production operations therefore presents a significant economic problem and safety risk.
It is known to use Low Dosage Hydrate Inhibitors (LDHIs) to prevent gas hydrate caused flow line blocking and equipment fouling problems. There are two types of LDHIs: Kinetic Hydrate Inhibitors (KHIs); and Anti-Agglomerants (AAs). KHIs inhibit the nucleation and/or growth of gas hydrate crystals in produced water whereas AAs prevent the agglomeration of hydrate crystals into problematic plugs.
The active part of most commercially available KHI formulations is a synthetic polymer. The most commonly used synthetic polymer is a water miscible poly-n-vinylamide such as polyvinylcaprolactam (PVCap). The active polymer typically makes up less than half of a KHI formulation with the remainder being water miscible polymer solvent such as a low molecular weight alcohol, e.g. methanol, ethanol or propanol, a glycol, e.g. monoethylene glycol (MEG) or a glycol ether, e.g. ethylene glycol monobutyl ether (EGBE) or 2-butoxyethanol. Dispersion of the solid polymer in the liquid solvent provides for ease of distribution of the KHI, for example by pumping of the KHI through pipelines to the inhibitor injection points. Furthermore the solvent acts as a synergist by enhancing the hydrate formation inhibiting properties of the polymer. The polymer is by far the most expensive part of KHI formulations.
KHIs offer many advantages over traditional approaches to hydrate inhibition. Nevertheless there are a number of problems associated with the use of KHIs including the following specific examples. In view of the non-biodegradable nature of many KHI polymers the disposal of KHI containing reservoir produced water is normally a significant issue where there is no reinjection of the produced water into the reservoir, e.g. where reinjection is impossible. Where produced water is treated KHI polymers are liable to foul treatment apparatus, such as MEG or methanol regeneration units. Where there is reinjection of produced water high reservoir temperatures can give rise to KHI polymer precipitation which is liable to block well perforations and rock pores and thereby reduce injection efficiency.
The present invention has been devised in the light of the inventors' appreciation of problems associated with the use of KHIs, including the problems mentioned above. It is therefore an object for the present invention to provide a method of treating aqueous fluid comprising a water miscible polymer, such as at least one Kinetic Hydrate Inhibitor (KHI). It is a further object for the present invention to provide aqueous fluid treatment apparatus which is configured to treat aqueous fluid comprising a water miscible polymer, such as at least one Kinetic Hydrate Inhibitor (KHI).